Method for the manufacture of bridge plugs



April 17, 1962 F. R. SEAVEY 7 3,029,681

METHOD FOR THE MANUFACTURE OF B RIDGE PLUGS Filed Sept. 24, 1958 INVENTOR. FREDERICK RSEAVEY A 7TORNE VS United States Patent ()fiice 3,6Z9fi3l Patented Apr. 17, 1962 This invention relates to explosive initiators and particularly to a method of making bridge plugs therefor.

Electric detonators usually consist of an explosive train contained in a cylindrical metal cup. This explosive train is initiated by a bridge wire embedded in a sensitive explosive within the case; the electric lead Wires for the bridge wire being supported by a sulfur plug which is positioned immediately above the ignition The detonator is then sealed by pouring a relatively deep layer of Waterproofing composition such as pitch in place while in a heated fluid condition and consequently rendered non-fluid by cooling. This pitch seal is normal ly held in place by a superposed layer of sulfur.

More recently, the metallic casing has been replaced with a plastic shell and the detonator has been closed by a molded plastic bridge plug carrying the lead Wires. The detonatoris then sealed by cementing the plastic bridge plug to the plastic case. Such detonators are markedly superior to metallic cased detonators in their susceptibility to sympathetic detonation, emission of shrapnel upon firing and in their resistance to mechanical shock as well as to undesirable electric stimuli. Also, generally speaking, they exhibit improved resistance to the ingress of water when compared with the conventional metallic blasting caps.

While plasticdetonators normally exhibit superior resistance to water, great care must be taken to insure a positive seal between the plastic bridge plug and plastic case, and also between the plastic plug and the metallic wires passing therethrough. Generally, satisfactory seals between the plastic components can be obtained through the utilization of a cement or solvent for these components but considerable difficulty has been encountered in providing a waterproof seal between the plastic plug and the metallic lead wires. In normal practice, the injection molded thermoplastic material about the wires is relied upon for the formation of the seal. Even under very closely controlled operating conditions, this is not completely satisfactory because the plastic material does not wet and has no afiinity for the metallic wires. Effective seals between the wire and plug material can be obtained but only by the utilization of additional procedural steps or materials, both of which increase the time and cost expended in the manufacture of such detonator components.

Therefore, it is an object of this invention to provide a new and novel blasting cap that overcomes the disadvantages and difiiculties of previously known blasting caps.

' Another object of this invention is a novel process for sealing plastic detonators. A more specific object of this invention is to provide an economical, simplified method for the manufacture of waterproof plastic detonator bridge plugs.

These and other objects are accomplished in accordance with this invention, generally stated, by forming a body of a polymerizable material into the shape of a bridge plug about a pair of lead wires and curing or polymerizing the material about the wires to form a rigid adherent bridge plug. More specifically, in accordance with this invention, a bridge plug is cast about the lead wires from a polymerizable liquid which is cured in place about the wires. Such casting can be executed in any type of a suitable mold or in a pro-formed cup positioned about the wires. It has been found that a monomeric or prepolymeric liquid wets the metallic wires as well as the plastic insulation about them and when the liquid is polymerized in contact with the lead wires, a definite bond is insured. Any suitable method of introducing the polymerizable liquid into the area about the lead wires can be employed. Preferably, a conventional casting method is followed in which the liquid is poured into a cavity through which the lead wires pass and the liquid is subsequently'polymerized. Alternately, conventional injectiontechniques can be employed followed by curing the polymerizable liquid. The particular curing operation will be dependent upon the type of polymerizable material used in the manufacture of the detonator plug. 'In most instances, the complete polymerization of the material can be effected at room temperature. However, it is preferred to accelerate the polymerization by means of elevated temperatures, catalysts, or a combination of these expedients.

Any plastic resinous material that is in liquid or flowable form in its monomeric or pre-polymeric state can be utilized in effecting the seal of the present invention. Polymeric preparations of this type that can be used include the epoxy resins, the polyamides, polyurethanes, and other liquid materials that can be polymerized by the application of moderate heat. Also, the present invention contemplates the utilization of self-curing or watercatalyzed polymeric preparations, such as alpha-cyanoacrylic acid and the like. When this type of material is used, no curing agent or additional catalyst is required, nor it is necessary to heat the injected liquid to eifect curing or polymerization thereof. While the most commonly used polymerizable materials in accordance with the invention are in liquid form, solid pre-polymers such as various types of epoxy resins that become fiowable on heating can be used with equal facility.

In accordance with this invention, the detonator lead wires can be formed of any of the conventional metals used for this purpose, such as iron, iron alloys, copper, copper alloys, aluminum and its alloys, or other suitable materials, and they can be provided with protective coatings of tin and the like. Also, the wires can be provided with any suitable plastic insulation such as ethyl cellulose, cellulose acetate, cellulose acetate butyrate, polyvinyl resins, polyarnides, polyethylene, and the like. Although a positive bond is formed between the lead wires and the polymeric material making up the bridge plug, that portion of the bared lead wires passing through the plug can be knurled, roughened, or otherwise treated so as to increase the surface area of the bond and pull strength of the unit. The lead wires generally define a straight path through the bridge plug but .in some instances can be kinked or even coiled within'the bridge plug.

The detonator bridge plugs made in accordance with this invention are of a generally cylindrical shape. They can, however, be slightly tapered to facilitate their assembly with a detonator case. The finished bridge plugs are particularly Well adapted for use in conjunction with plastic detonator cases but can also be used with metallic cased detonators.

The manner in which the objects of the present invention are accomplished will be more readily appreciated by those skilled in the art by reference to the following illustrative embodiments of the invention taken together with the drawing in which:

FIGURE 1 is a sectional side elevation of casting apparatus utilized in carrying out the processes of this invention;

FIGURE 2 is a cross-sectional view taken along the line 11-11 of FIGURE 1; and

FIGURE 3 illustrates another embodiment of the present invention.

As shown in FIGURES 1 and 2, die block 1 is provided with mold cavities 2, each having a pair of perforations 3 in the bottom thereof. These perforations are sized so as to form a snug or sliding fit with the bared portions of the lead wires. Thus, when the bared portions of lead wires 4 are positioned as shown in FlGURE 1 within the perforations 3, the mold cavities 2 are provided with effectively closedbottoms. It is preferred that a portion of the plastic insulation 5 be removed from a segment of the wires adjacent their ends so that the insu lation will terminate within the bridge plug. As illustrated in FIGURE 1, after the lead wires had been thus positioned, the mold cavities 2 were filled with an epoxy resin pre-polymer containing about 8% diethylene triarnine as a cross-linking catalyst. The particular resin used was Shell Epon Resin No. 828 but it will be readily appreciated that many of the commercially available epoxy resin preparations can also be used. The resin was then cured by heating the die block containing the epoxy resin and the pairs of detonator lead wires to a temperature of about 45 C. for about three days. After this curing process, the epoxy resin was completely polymerized and the finished plugs removed from the mold cavity. Detonator plugs prepared in this manner exhibited greater pull strength than conventionally molded plastic bridge plugs and in contrast to such conventionally prepared plugs, the epoxy resin polymerized in place about the lead wires formed astrong positive bond'with the lead wires, thus eliminating any possibility of water entering between the plastic material of the bridge plug and the lead wires.

In the embodiment shown in FIGURE 3 of the drawing, a pair of detonator lead wires 3 provided with insulating sheaths 5 of ethyl cellulose are positioned with a pre-formed ethyl cellulose cup having a substantially cylindrical sidewall 6 and a flat bottom 7 having two perforations through which the lead wires pass. Although, in accordance with this example, the plastic cup was preformed by injection molding, it will be readily appreciated that any other method of forming the cup can be utilized. After the lead wires had been positioned within the plastic cup as shown in this figure, cavity 8 of the cup was filled with the epoxy resin used in the foregoing example. These assemblies were then cured as described above 45 C. for about three days. Detonator plugs made in this fashion possessed the same advantageous characteristics as those described above. When the plastic cup is made of rubber or other resilient material, the resultant detonator plugs are particularly well suited for use with metallic cased detonators. The layer of rubberlike material thus provided between the plastic plug and the metallic case permits the case to be crimped into the plug without destroying any of the effectiveness of the wire-to-plug seal achieved in accordance with this invention.

The above embodiments are directed to detonator bridge plugs prepared from epoxy resins, but it is to be understood that other types of liquid materials capable of being polymerized can be utilized with equal facility. Also, the die block having cylindrical mold cavities can be replaced by split mold apparatus, injection molding machines, or any other conventional mechanisms used in the plastic industry. The process of the present invention can also be used for the manufacture of detonator bridge plugs having various shapes, shoulders, and appendages, as well as the cylindrical bridge plugs described above.

The term detonator or electric initiator is used herein in a generic sense to encompass all types of electrically activated explosive initiators including detonators, blasting caps, squibs, deflagrating devices, and the like.

Since variations in the specific embodiments which have been described may be made within the spirit and scope of this invention, the detailed description is not to be considered as limitative except in the light of the appended claims.

Having thus described the invention, what is claimed is:

1. A method for the manufacture of bridge plugs for electric blasting caps comprising forming a plastic cup with a substantially cylindrical sidewall and a substantially flat bottom having a pair of openings therethrough, providing a pair of plastic-insulated lead wires having the insulation removed from a portion of the wires adjacent one end thereof, positioning the uninsulated portion of the lead wires in the openings in the bottom of the cup, filling the cup with a polymerizable liquid, and polymerizing the liquid in situ, thus forming a positive bond between the lead wires and the polymerized material.

2. A method for the manufacture of bridge plugs for electric blasting caps comprising forming a plastic cup with a substantially cylindrical sidewall and a substantially flat bottom having a pair of openings therethrough, providing a pair of plastic-insulated lead wires having the insulation removed from a portion of the wires adjacent one end thereof, positioning the uninsulated portion of the lead wires in the openings in the bottom of the cup, filling the cup with a polymerizablc material rendered flowable by heating and polymerizing the material in situ, thus forming a positive bond between the lead wires and the polymerized material.

References Cited in the file of this patent UNITED STATES PATENTS 2,515,238 Lefren July 18, 1950 2,685,835 Noddin Aug. 10, 1954 2,687,667 Gunther Aug. 31, 1954 2,695,562 Mulqueeny Nov. 30, 1954 2,695,563 Mulqueeny Nov. 30, 1954 2,777,389 Lawrence Jan. 15, 1957 

